In this situation I would say that tipping would be considered odd/unusual.

You would usually tip the waiter/waitress in a restaurant or pub if you were paying at the table. I.e. they presented you with a bill at the end of your meal; the tip being in response to receiving what you considered to be good service.

If you were paying in advance for your meal at the bar then you don't know if you will receive good service or not.

However, if you *have* paid in advance you *could* hand a tip over after your meal to the barman/barmaid on your way out as a thank you. 

In either case who actually ends up with this tip money varies a great deal from place to place: [Where does your tip really go? The truth about restaurant gratuities][1], which is one reason why people can be reluctant to tip at all in the UK (especially in pub/restaurant chains)

Note that tipping in the UK *is* optional unlike the USA where you are expected to tip 15-20% as the norm, regardless of whether you paid before or after your meal.